Method and apparatus for determining transmission wavelengths for lasers in a dense wavelength division multiplexer

ABSTRACT

The method and apparatus operates to calibrate a transmission laser of the dense wavelength division multiplexer (DWDM). In one example, the transmission laser is a widely tunable laser (WTL) to be tuned to one of a set of International Telecommunications Union (ITU) transmission grid lines for transmission through an optic fiber. The WTL is tuned to the ITU grid using an etalon and a gas cell having acetylene, hydrogen cyanide or carbon dioxide. Initially, the absolute transmission wavelengths of the WTL are calibrated by routing an output beam from the WTL through the etalon and through the gas cell while varying tuning parameters of the WTL to thereby generate an etalon spectrum and a gas absorption spectrum both as functions of the tuning parameters. The etalon and gas absorption spectra are compared, along with input reference information specifying gas absorption as a function of absolute wavelength, to determine the absolute transmission wavelength for the WTL as a function of the tuning parameters. The WTL is then tuned to align the transmission wavelength of the WTL to an ITU transmission grid line.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The invention generally relates to dense wavelength divisionmultiplexers (DWDM) and in particular to a technique for determiningtransmission wavelengths of lasers of the DWDM as a function of lasercontrol tuning parameters.

II. Description of the Related Art

A DWDM is a device for simultaneously transmitting a set of discreteinformation channels over a single fiber optic transmission line. Aconventional fiber optic transmission line is capable of reliablytransmitting signals within a bandwidth of 1280 to 1625 nanometers (nm),the “low loss” region for silica fiber. Within that overall bandwidth,the International Telecommunications Union (ITU) has defined varioustransmission bands and specified certain transmission channel protocolsfor use within each transmission band. One example of a transmissionband is the ITU “C” band, which extends 40 nm from 1525 nm to 1565 nm.Within the C band, specific transmission channel protocols of 40, 80, or160 discrete channels are defined and, for each protocol, the ITU hasdefined a grid of transmission wavelengths, with each line correspondingto an acceptable transmission wavelength. For the 40 channel protocol,the corresponding ITU grid has 40 lines with channel spacing of 0.8 nm;for the 80 channel protocol, the corresponding ITU grid has 80 lineswith channel spacing of 0.4 nm; and so forth. The protocols have beendefined to ensure that all DWDM transmission and reception equipment arefabricated to operate at the same wavelengths. Other exemplary ITUtransmission bands are the S- and L-bands.

To simultaneously transmit the set of channels on a fiber optic cable,the DWDM employs a set of individual distributed feedback (DFB) lasers,with one DFB laser per channel. FIG. 1 illustrates a DWDM 100 havingforty individual DFB lasers 102 for transmitting optical signals via asingle optic fiber 104. An optical multiplexer 106 couples signalsreceived from the individual DFBs via a set of intermediate optic fibers107 into output optic fiber 104. Each DFB laser transmits at a differentwavelength of the forty channel ITU C band. This enables forty separatechannels of information to be transmitted via the single optical fiber104 to a de-multiplexer (not shown) provided at the far end of theoptical fiber.

To permit the DWDM to transmit forty separate channels simultaneously,each individual DFB must be tuned to a single ITU transmission channelwavelength. A DFB laser can be tuned only within a narrow wavelengthband, typically about 4 nm in width. Hence, for the 40 channel protocolof the ITU C band having 0.8 nm transmission line spacing, the typicalDFB can only be tuned to one of a few adjacent lines out of the total of40 lines of the ITU grid. Traditionally, each individual DFB laser ismanually calibrated at the factory to emit at one of the correspondingITU transmission lines. This calibration is achieved by adjusting thelaser operating temperature and current to obtain the desiredwavelength. The laser is then, in some implementations, locked to thetarget wavelength by routing the output beam from each DFB laser througha corresponding manually-tunable etalon. (The etalons are not shown inFIG. 1.) A manually-tunable etalon is an optical device which produces aperiodically-varying transmission spectrum as a function of laserwavelength. By tilting the etalon relative to the DFB laser beam path, atransmission peak of the etalon can be made coincident with the targetITU channel. The wavelength of a etalon transmission peak is calibratedto one of the ITU transmission lines by manually adjusting the angle ofthe etalon while monitoring the wavelength output from the etalon usingan optical wavelength analyzer. The angle of the etalon is adjusteduntil the output wavelength is properly aligned with one of the ITUtransmission lines, then the etalon is mounted in place in an attempt tolock the output wavelength of etalon to the selected ITU transmissionline. This is a difficult and time consuming process requiring skilledtechnicians. Calibration of all forty DFB lasers of a single DWDM can bequite expensive. Mechanical or thermal drift of the etalon over timeoften moves the transmission peak away from the target ITU channel whichrequires recalibration.

Once the DFB lasers of a single DWDM are properly aligned with the ITUgrid, the DWDM may then be used for transmitting signals over a fiberoptic line, such as for transmitting digital data over computer networks(i.e., the Internet) or for transmitting television signals from atelevision network to one of its affiliates. A single DWDM must beprovided for use with each fiber optic line employed for DWDMtransmissions and hence a single customer installation, such as atelevision broadcast center, may require many, many DWDMs. If one of theDFB lasers within a DWDM drifts from its corresponding ITU transmissionline or otherwise malfunctions, the entire DWDM typically needs to bereplaced requiring the malfunctioning DWDM to be returned to the factoryto be re-calibrated or otherwise fixed. As a result, the cost ofmaintaining a set of DWDMs is often substantial. To help remedy thisproblem, some DWDMs are provided with an additional widely tunable laser(WTL) which can be tuned separately to any one of the ITU grid lines.Hence, if one of the DFB lasers malfunctions, the single WTL can betuned to the corresponding transmission wavelength of the DFB to therebypermit the DWDM to continue to operate. Additional WTLs can be suppliedwith a DWDM to accommodate the failure of two or more DFB channels, andsuch “sparing” is a major advantage a WTL over a DFB. However, the WTLcannot simply and accurately be tuned to any target ITU channel at acustomer installation and must be calibrated at the factory foroperation at a specific channel.

Another problem associated with employing DFB lasers within DWDMs isthat, because each DFB laser can only be tuned within a narrow range ofabout 4 nm, each DFB laser can only be calibrated to one of a fewadjacent ITU transmission wavelength lines. It may also sometimes bedesirable to configure the DWDM to use many lasers for transmitting at asingle ITU transmission line to provide more bandwidth on that channel.When using DFB lasers, no more than two or three of the lasers can becalibrated to a single ITU transmission line. Hence, in some DWDMs, WTLsare used exclusively instead of DFB lasers, thus permitting any of thelasers to be manually calibrated at the customers installation totransmit on any of the ITU transmission lines. Although the use of WTLsremedies many of the problems associated with using DFB lasers, WTLs aredifficult and expensive to fabricate and initially calibrate, and aresusceptible to wavelength drift requiring frequent recalibration at thecustomers installation by trained technicians and hence necessitatinghigh overall installation and maintenance costs.

Thus, whether using DFB lasers or WTLs within a DWDM, significantproblems arising in achieving and maintaining proper wavelengthcalibration of the lasers to permit reliable operation of the DWDM.Accordingly it would be desirable to provide an efficient method andapparatus for calibrating transmission lasers within a DWDM and it is tothat end that the invention is primarily directed.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, a method andapparatus is provided for calibrating a laser using an etalon and a gasabsorption cell containing a gas of known light absorptioncharacteristics. In accordance with the method, an output beam from thelaser is routed through the etalon while the laser is tuned through arange of tuning parameters to produce an etalon transmission spectrum asa function of the laser tuning parameters. The output beam from thelaser is also routed through the gas cell while the laser is tunedthrough the range of tuning parameters to produce a gas absorptionspectrum as a function of the laser tuning parameters. The etalontransmission spectrum and the gas absorption spectrum are detected andthen compared to determine the absolute transmission wavelength of thelaser as a function of the laser tuning parameters. Thereafter, thelaser can be set to transmit at a selected transmission wavelength byusing the tuning parameters that correspond to the selected wavelength.

In an exemplary embodiment, the invention is implemented as a hand-heldwavelength mapper for use with a transmission WTL of a DWDM to be tunedto an ITU transmission grid line for transmission through an opticfiber. The laser is tuned using a single tuning parameter, which may bean input voltage or current. The known gas, which may be hydrogencyanide, acetylene or carbon dioxide, is contained within a sealed gascell of the wavelength mapper. A portion of the output beam of the laseris split off and routed separately through the etalon and the gasabsorption cell to two separate detectors for detecting both an etalontransmission spectrum and a gas absorption spectrum.

The absolute transmission wavelengths of the WTL as a function of theinput voltage or current WTL tuning parameters are determined asfollows. The detected etalon transmission spectrum has transmissionpeaks that are separated by a precisely known wavenumber (wavenumber isthe number of wavelengths of laser light per cm, and so is inverselyproportional to wavelength) determined by the construction material,physical dimension, and optical properties of the etalon. Thiswavenumber “comb” is exploited to determine relative wavenumbers for thetuning parameters. To this end, transmission lines in the detectedetalon transmission spectrum are identified, a relative wavenumber isassigned to the tuning parameter corresponding to each consecutiveetalon transmission line, and then relative wavenumbers are assigned toeach intermediate value of the tuning parameters by interpolatingbetween the transmission lines. Next, the detected gas absorptionspectrum, which is a function of WTL tuning parameters, is converted toa modified gas absorption spectrum, which is a function of relativewavenumber, by assigning relative wavenumbers to each value of thedetected gas absorption spectrum based on the associated tuningparameter. Then, the modified gas absorption spectrum is compared withan input gas absorption spectrum, which is a function of absolutewavenumber, to determine corresponding absolute wavenumbers for eachvalue of the tuning parameters. This is achieved by inputting apredetermined gas absorption spectrum specifying absorption as afunction of absolute wavenumber; correlating the modified gas absorptionspectrum, which is a function of relative wavenumber, with the inputabsorption spectrum, which is a function of absolute wavenumber, todetermine an offset between relative wavenumbers and the absolutewavenumbers; and then adjusting the relative wavenumbers associated witheach value of the tuning parameters by the offset value to provide anabsolute wavenumber for each value of the tuning parameters. If needed,the wavenumbers can be easily converted to wavelengths or frequencies.In this manner, the absolute transmission wavelength, frequency, orwavenumber of the WTL is thereby determined as a function of the tuningparameters.

Hence, by tuning the output wavelength of the WTL using an etalon incombination with a gas absorption cell, the WTL can be quickly, easilyand precisely set to a selected ITU transmission grid line at acustomers installation. The tuning process can be periodically repeatedto maintain precise tuning of the WTL despite possible temperature ormechanical drift. Thus overall installation and maintenance costsassociated with DWDMs can be significantly reduced. By providing preciseand reliable tuning of the lasers of the DWDM, the invention alsofacilitates the use of a greater number of transmission channels, suchas 80, 160 channels, or more.

In general, any laser tunable using any set of input tuning parameters,such as various combinations of input analog or digital signals, can beused with the invention so long as an appropriate gas absorptionreference is available. The laser is simply scanned through its fullrange of tuning parameters to enable determination of the absoluteoutput wavelength of the laser as a function of any combination of thetuning parameters.

In accordance with a second aspect of the invention, a method andapparatus is provided for locking a laser to a transmission wavelengthusing an etalon and a gas absorption cell. The etalon used to map theoutput wavelengths of the laser is a temperature-controlled etalon. Theaforementioned wavelength mapping steps are performed to determine theabsolute wavelength of the laser as a function of the laser tuningparameters. Tuning parameters are applied to the laser to tune the laserto a selected transmission wavelength, such as an ITU channelwavelength. Additionally, a temperature offset is applied to the etalonto vary the wavelengths of the transmission peaks of the etalon untilone of the transmission peaks is precisely aligned with the selectedwavelength. Any drift of the laser output from the selected wavelengthis detected and the tuning parameters applied to the laser areautomatically adjusted to compensate for the drift. Thus, a feedbackloop is provided which keeps the main output beam locked on a selectedtransmission channel despite possible variations in the outputcharacteristics of the laser. In an exemplary embodiment, the inventionis implemented as a wavelength locker for mounting to a transmission WTLof a DWDM to be tuned to an ITU transmission grid line for transmissionthrough an optic fiber. In one specific implementation, the temperatureoffset is applied to the etalon by applying an electrical current to theetalon.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, objects, and advantages of the invention will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings in which like reference charactersidentify correspondingly throughout and wherein:

FIG. 1 illustrates a DWDM configured in accordance with the prior art.

FIG. 2 illustrates a DWDM along with a handheld, portable wavelengthmapper provided in accordance with a first exemplary embodiment of theinvention, with the wavelength mapper provided for automaticallydetermining the transmission wavelengths of the lasers of the DWDM as afunction of tuning parameters of the lasers.

FIG. 3 illustrates the wavelength mapper of FIG. 2.

FIG. 4 illustrates a method performed by the wavelength mapper of FIG.3.

FIG. 5 illustrates an exemplary etalon transmission spectrum detected bythe method of FIG. 4, scaled as a function of a laser voltage tuningparameter.

FIG. 6 illustrates an exemplary gas absorption spectrum detected by themethod of FIG. 4, also scaled as a function of the laser voltage tuningparameter.

FIG. 7 illustrates an input reference gas absorption spectrum.

FIG. 8 illustrates an individual DWDM laser along with a wavelengthlocker provided in accordance with a second exemplary embodiment of theinvention, with the wavelength locker provided for automaticallydetermining the transmission wavelengths of the laser and for lockingthe wavelength of the laser to a selected ITU transmission wavelength byusing a voltage-controlled etalon in a feedback loop.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

With reference to the remaining figures, exemplary embodiments of theinvention will now be described.

FIG. 2 illustrates a DWDM 200 having forty individual WTLs 202 fortransmitting optical signals on forty ITU C-band channels via a singleoptic fiber (not shown). In FIG. 2, an optic fiber output from aselected WTL is connected via a fiber optic line 203 to the input of ahand-held, portable wavelength mapper 204 configured for automaticallydetermining the transmission wavelength of the WTL as a function of WTLtuning parameters, such as a WTL control voltage or current. Output fromthe wavelength mapper to the selected WTL of DWDM is via a control line205. Although a forty channel DWDM is shown, in other implementations80, 160, or more WTLs are provided. Also, other lasers may be employedin the alternative, such as DFB lasers, provided their tuning range issufficient to record a minimum number of gas absorption lines (>5).

To permit the DWDM to transmit the forty separate ITU channelssimultaneously, each individual WTL of the DWDM must be precisely tunedto a single ITU transmission channel wavelength. For an example whereinthe WTLs are tuned by applying a control voltage to the WTL, a separatevoltage level is associated with each ITU wavelength. The wavelengthmapper operates to determine the resulting transmission wavelength foreach WTL for values of the control voltage throughout an entire voltagetuning range. This process is performed sequentially for each of theforty WTLs to generate a separate wavelength vs. voltage map for eachWTL. Thereafter, any particular WTL can be tuned to any selectedtransmission wavelength merely by accessing the corresponding wavelengthvs. voltage map to determine the appropriate control voltage. Typically,the WTLs are set to selected ITU C-band channels, but can be set to anyselected wavelength. Wavelength mapping is performed when a new WTLlaser is fabricated and its tuning parameters must be determined, andwhen an installed WTL must be accurately tuned to another ITU channel inthe field by field service personnel.

FIG. 3 illustrates pertinent internal components of wavelength mapper204. The wavelength mapper receives an input optical beam from one ofthe WTLs of the DWDM (FIG. 2) via optic fiber 20. The input beam iscollimated using a collimator 207 then split using a splitter 208, withone portion of the beam being routed through a gas cell 210 and anotherportion being routed through an etalon 212. The gas cell contains gashaving a known absorption spectrum with numerous absorption lines in theoptical bandwidth in which the laser is to be tuned. For a laser to betuned within the ITU C- and S-bands, acetylene is appropriate, withcarbon dioxide being suitable for the L-band. The etalon is configuredto provide numerous transmission lines within the optical bandwidth inwhich the laser is to be tuned. The etalon, as with all etalons,provides transmission lines (or fringe peaks) equally spaced in terms ofwavenumbers. (A wavenumber is 10,000/(wavelength in microns) and therebycan easily be converted to wavelength or frequency). For use with aforty channel ITU C-band DWDM, the etalon is preferably configured toprovide at least five hundred transmission peaks in the C-band.

A first optical detector 214 detects a beam emergent from the gas celland a second optical detector 216 detects a beam emergent from theetalon. Signals detected by the detectors are routed into amicrocontroller 218 for processing therein. The microcontroller is alsoconnected to the DWDM via control line 205 to control the selected WTLof the DWDM to scan through the entire ITU C-band. In other words, themicrocontroller varies the voltage or current input to the WTLthroughout an entire input range to thereby vary the transmissionwavelength of the WTL throughout the entire ITU C-band. As a result, thetwo optical detectors both receive an entire spectrum of optical signalscovering the entire ITU C-band. The detector coupled to the etalondetects an etalon spectrum having etalon transmission lines therein. Thedetector coupled to the gas cell detects a gas absorption spectrumhaving gas absorption lines therein. The microcontroller also inputs areference gas absorption spectrum for the gas contained within the gascell wherein the reference absorption spectrum specifies the absolutewavenumber, wavelength or frequency for each of the absorption lines ofthe gas. The microcontroller processes the detected etalon and gasabsorption spectra in combination with the reference gas spectrum todetermine the transmission wavelengths of the WTL as a function of thevoltage or current tuning parameter applied to the WTL to thereby mapthe wavelengths of the WTL. The wavelength map is stored for subsequentuse in setting the WTL to transmit at any selected wavelength, such asat one of the ITU C-band channels.

The manner by which the wavelength mapper generates a wavelength vs.tuning parameter map for a WTL or other laser will now be described ingreater detail with reference to FIGS. 4-7. Initially, at step 300 ofFIG. 4, the wavelength mapper routes an output beam of the laser throughthe etalon and through the gas cell while tuning the laser through acomplete range of tuning parameters to generate an etalon transmissionspectrum and gas absorption spectrum. In one specific example, for alaser tuned by a control voltage ranging from 0.0 to 40.0 volts, thewavelength mapper incrementally increases the voltage from 0.0 to 40.0volts by voltage increments of 0.0000610352 volts to generate etalon andgas absorption spectra each with 65536 data points. The etalon and gasabsorption spectra are detected at step 306 and stored in separate dataarrays by the wavelength mapper. A section of an exemplary etalonspectrum 302 for an etalon having a peak spacing of about 6.6 gigahertz(GHz) is shown in FIG. 5. Preferably, however, an etalon with a peakspacing of 8 GHz is used. A section of an exemplary gas absorptionspectrum 304 for acetylene is shown in FIG. 6. Both spectra are scaledby voltage. For each data point, the wavelength mapper also stores thecorresponding data point number in a data array. Hence, the detectedetalon and gas absorption spectra are both recorded as functions ofvoltage, not wavelength or frequency which is as yet unknown.

Continuing with FIG. 4, the wavelength mapper then processes thedetected etalon spectrum at step 308 to identify and locate transmissionpeaks therein. The peaks are located by determining first and secondderivatives of the etalon spectrum as a function of data point and byapplying polynomial fitting in the local peak areas in accordance withconventional techniques. The location of each peak is specified by itscorresponding fractional data point number. Note that the peaks are notequally spaced in terms of the data points. Rather, the peaks aregenerally non-linearly distributed among the data points. In any case,once the peaks are identified and located, the wavelength mappersequentially assigns relative wavenumbers to each of the transmissionpeaks beginning with 1 and proceeding to the last detected peak. In theexample of FIG. 5 (which shows only a very small section of the etalonspectrum), there are 37 peaks and hence the peaks may be numbered frome.g. 400 to 437. The relative wavenumbers generated by this process arestored in the etalon spectrum data array at the fractional data pointcorresponding to the voltage peak, and interpolated to the integer pointnumbers surrounding the peak. For example, if the 403rd transmissionpeak is found at data point 50788.56 out of the 65536 data points, thenrelative wavenumber 403 is assigned to fractional data point 50788.56.Relative wavenumbers for integer data points 50788 and 50789 areobtained by interpolation and stored in the etalon data array.Similarly, if the 404th transmission peak is found at data point50823.17 out of the 65536 data points, then relative wavenumbers 404 isstored in association with fractional data point 50823.17. Fractionalrelative wavenumbers for the adjacent integer points 50823 and 50824 areassigned by interpolation. The relative wavenumbers can be assigned tothe transmission peaks of the etalon spectrum sequentially because thepeaks are generated by an optical etalon which, by virtue of its opticalproperties, produces peaks substantially equally spaced in wavenumber.Hence, even though the peaks are not equally spaced as a function of thedata points or as a function of laser input voltage, the peaks arenevertheless equally spaced as a function of relative wavenumber, andsequential wavenumbers can be reliably assigned. The wavenumbers arereferred to herein as relative wavenumbers because the absolutewavenumber (and hence the absolute wavelength or wavelength) is not yetknown.

Thus, upon completion of step 308, relative wavenumbers have beenassigned only to those integer data points in the etalon spectrum arraythat correspond to the closest etalon transmission peak. At step 310,the wavelength mapper interpolates between the peaks to assignfractional wavenumbers to each intermediate data point. For the examplewherein the relative wavenumber 403 falls between data points 50788 and50789, and relative wavenumber 404 falls between integer data points50823 and 50824, the wavelength mapper interpolates between the assignedfractional wavenumbers to data points 50789 through 50822. In onespecific example, as a result of the interpolation, data point 50789 maybe assigned a relative wavenumber of 6471.5600; data point 50790 may beassigned a relative wavenumber of 6471.5625; and so on. In this manner,interpolation is preformed to assign fractional relative wavenumbers toeach remaining value in the etalon spectrum data array. Note that thefractional wavenumbers are not necessarily evenly distributed betweeninteger wavenumbers. Rather, as a result of the interpolation, thefractional wavenumbers may be assigned non-linearly. Thus followinginterpolation, each integer data point of the etalon array has arelative wavenumber associated therewith. The relative wavenumbers arestored along with the corresponding voltage values in the etalonspectrum data array to thereby provide a relative wavenumber for eachdata point.

At step 312, the relative wavenumbers generated for each data point ofthe etalon array are used to re-scale the gas spectrum data array. Tothis end, the relative wavenumber of each data point of the etalonspectrum array is assigned to the corresponding data point of thedetected gas absorption spectrum array. At this point a relativewavenumber scale exists both for the etalon transmission spectrum andthe gas absorption spectrum. However, the relative wavenumber scale isnot linear because of the non-linear tuning properties of the laser.

At step 318, the wavelength mapper inputs a reference gas absorptionintensity spectrum for the gas of the gas cell, wherein the referencespectrum is scaled according to absolute wavenumber, rather thanrelative wavenumber. FIG. 7 illustrates a portion of an exemplaryreference gas intensity spectrum 322 for acetylene. This spectrum isgenerated synthetically using the know frequencies and intensities ofthe reference gas, which are known to high accuracy through publishedlaboratory measurements and verification by the National Institute ofStandards and Technology (NIST). The reference spectrum is input as adata array of equal size to the modified gas absorption data array, e.g.65536 data points. At step 320, the wavelength mapper autocorrelates theintensity pattern of the modified detected gas absorption spectrum,which is a function of relative wavenumber, with the intensity patternof the input reference spectrum, which is a function of absolutewavenumber, to determine any offset therebetween. An appropriateautocorrelation technique, modified as needed, may be found in“Correlation-based Technique for Automated Tunable Diode Laser ScanStabilization”, Randy May, Rev. Sci. Instrum. 63 (5), May 1992. As asecond iteration of the process, the etalon transmission peak spacing(the etalon “free spectral range”, or FSR) is more precisely determinedfrom the known gas spectrum line positions, and the wavenumber mappingprocess is repeated to improve accuracy.

Thus, following step 318, the wavelength mapper stores the modifieddetected gas intensity spectrum generated at step 312 and the referencegas intensity spectrum input at step 318. The two spectra are similarbut are offset from one another. Alternatively, at step 320,autocorrelation may be performed to determine the shift of the spectrawith respect to one another until the spectra are aligned, thuspermitting the amount of shift or offset to be determined. The offsetrepresents the offset between the relative wavenumbers and theircorresponding absolute wavenumbers. At step 324, the relativewavenumbers of the various arrays are adjusted using the offset toconvert the relative wavenumbers to absolute wavenumbers. Once theabsolute wavenumbers are known, an absolute wavelength or frequency isassigned at step 326 to each of the control voltage values stored in theetalon spectrum array.

Although the wavelength mapper has been described with respect to anexemplary embodiment wherein the laser is controlled by a single voltagecontrol signal, in general, any laser can be used with the invention solong as an appropriate gas absorption reference is available and thelaser is tunable via a set of input tuning parameters, such as variouscombinations of input analog or digital signals. The laser is simplyscanned through its full range of tuning parameters to enabledetermination of the absolute output wavelength of the laser as afunction of any combination of the tuning parameters. The resultingwavelength vs. tuning parameters map is therefore a multi-dimensionalmap having a unique wavelength for each combination of tuningparameters. For some lasers tunable with two parameters, it may besufficient to set a first tuning parameter to a single constant valuewhile varying a second tuning parameter, then set the second tuningparameter to a single constant value while varying the first tuningparameter. In other cases, it may be necessary to tune the laser throughevery possible combination of the two parameters to account fornon-linear effects. For any given laser, routine experimentation can beperformed to determine the specific manner with which the tuningparameters are to be varied.

What has been described thus far is a wavelength mapper which operatesto generate a map of wavelength vs. tuning parameters for a laser,particularly one in a DWDM. In the following, a wavelength locker isdescribed which automatically sets the laser to a selected wavelength byusing a wavelength map, and then locks the laser wavelength using anetalon transmission peak that has been temperature or voltage tuned tothe selected ITU channel. As many of the features of the wavelengthlocker are the same as the wavelength mapper described above, onlypertinent differences will be described in detail.

FIG. 8 illustrates pertinent internal components of a wavelength locker400 for use with a WTL 401. The wavelength locker receives the outputfrom WTL 401 via an optical fiber splitter 402. The laser beam input tothe wavelength locker is initially of unknown wavelength. Inside thewavelength locker, a second splitter 404 splits the beam in two with oneportion routed through a gas cell 408 and the other portion reflectedfrom a mirror 409 and then routed through an etalon 410. Separatedetectors 416 and 418 record the transmission spectra of the gas celland the etalon as with the wavelength mapper. A microcontroller 420varies control parameters input to the WTL along a control line 422 togenerate a spectrum having both etalon transmission peaks and gasabsorption lines. The recorded spectra are fed into the microcontrollerfor processing to generate a wavelength vs. WTL tuning parameter mapusing the techniques described above. Once the wavelength vs. WTL tuningparameter map has been generated, the microcontroller looks up the WTLtuning parameter corresponding to a selected wavelength, such as an ITUchannel wavelength, then applies the WTL tuning parameter along controlsignal along line 422 to tune the WTL to the selected transmissionwavelength. Additionally, the microcontroller adjusts a temperaturecontrol set point to the etalon via a control line 424 to vary thewavelengths of the transmission peaks of the etalon until one of thetransmission peaks, as detected by detector 416, is precisely alignedwith the selected output wavelength. The microcontroller then locks theoutput wavelength of the WTL to the selected wavelength by monitoringthe etalon transmission peak that is aligned with the selectedwavelength. To this end, the micro-controller detects any drift of theWTL relative to the etalon transmission line as detected by detector 418and adjusts the tuning parameters applied to the WTL via control line422 to compensate for the drift. In other words, a negative feedbackloop is provided which keeps the main output beam locked on a selectedetalon transmission channel despite possible variations in the outputcharacteristics of the WTL.

Alternatively, gas cell 408 and etalon 410 are provided along a commonoptical path and a single detector is provided to detect the etalon andthe gas absorption spectra simultaneously. Although the resultingspectra has both etalon peaks and ga absorption lines, the etalon peaksand the gas absorption lines do not significantly interfere with oneanother and hence the wavelength mapping process performed above can beperformed. In this regard, the etalon peaks represent about 30% changesin transmission, whereas the gas lines represent only about 1% usingsecond harmonic detection. Thus, the gas lines represent a very smallperturbation to the etalon spectrum and do not interfere with the etalonwavenumber locking procedure, but are strong enough to permit theautocorrelation procedure without significant errors.

The exemplary embodiments have been primarily described with referenceto block diagrams illustrating pertinent components of the embodiments.It should be appreciated that not all components of a completeimplementation of a practical system are necessarily illustrated ordescribed in detail. Rather, only those components necessary for athorough understanding of the invention have been illustrated anddescribed in detail. Actual implementations may contain more componentsor, depending upon the implementation, fewer components.

The description of the exemplary embodiments is provided to enable anyperson skilled in the art to make or use the invention. The invention isnot intended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein. For example, one of ordinary skill in the artwill appreciate that the current invention may be used for thecalibration of wavelength demultiplexers at the receiving end of DWDMsystems as well as for all applications using tunable photonicsequipment (i.e., add/drop filters, provisioning to increase bandwidth).Consequently, the scope of the present invention should not be limitedby the particular embodiments discussed above, but should be definedonly by the claims set forth below and equivalents thereof.

What is claimed is:
 1. A method for calibrating a laser using an etalonand a gas absorption cell containing a gas of known light absorptioncharacteristics, the method comprising the steps of: routing an outputbeam from the laser through the etalon while tuning the laser through arange of discrete laser-tuning parameters where said tuned parametersare selected from the group consisting of current, voltage, andtemperature to produce an etalon transmission spectrum; routing anoutput beam from the laser through the gas cell while tuning the laserthrough the range of discrete laser tuning parameters to produce a gasabsorption spectrum; detecting and storing peaks of the etalontransmission spectrum and the gas absorption spectrum; and comparing thestored detected etalon transmission spectrum with the stored detectedgas absorption spectrum to determine the absolute transmissionwavelength of the laser, thereby providing references for preciselytuning the laser.
 2. The method of claim 1 further including the stepsof: selecting a wavelength for transmission; determining particulartuning parameters needed to tune the laser to the selected transmissionwavelength based on the absolute transmission wavelength of the laser;and tuning the transmission laser to the selected wavelength using theparticular tuning parameters.
 3. The method of claim 2 wherein the laseris tuned based on an input voltage and wherein the step of determiningthe particular tuning parameters needed to tune the laser to theselected wavelength is performed to determine the input voltage.
 4. Themethod of claim 2 wherein the wavelength for transmission is selectedfrom a group of predetermined International Telecommunication Industry(ITU) fiber optic transmission grid lines.
 5. The method of claim 1wherein the step of comparing the detected etalon transmission spectrumwith the detected gas absorption spectrum to determine the absolutetransmission wavelength of the laser as a function of the laser tuningparameters includes the steps of: comparing the detected etalontransmission spectrum, which is a function of tuning parameters, withrelative wavenumbers of known etalon transmission lines to determinecorresponding relative wavenumbers for the tuning parameters; convertingthe detected gas absorption spectrum, which is a function of lasertuning parameters, to a modified gas absorption spectrum, which is afunction of relative wavenumber, based on the corresponding relativewavenumbers of the tuning parameters; and comparing the modified gasabsorption spectrum, which is a function of relative wavenumber, with aninput gas absorption spectrum, which is a function of absolutewavenumber, to determine corresponding absolute wavenumbers for thetuning parameters such that the absolute transmission wavelength of thelaser as a function of the laser tuning parameters is therebydetermined.
 6. The method of claim 5 wherein the step of comparing thedetected etalon transmission spectrum with relative wavenumbers of knownetalon transmission lines to determine corresponding relativewavenumbers for the tuning parameters includes the steps of: identifyingtransmission lines in the detected etalon transmission spectrum;assigning a relative wavenumber to the tuning parameter value associatedwith each consecutive etalon transmission line; and interpolatingbetween the transmission lines to assign relative wavenumbers to eachintermediate value of the tuning parameters such that the correspondingrelative wavenumbers for the tuning parameters are thereby determined bythe relative wavenumbers.
 7. The method of claim 6 wherein the step ofconverting the detected gas absorption spectrum to the modified gasabsorption spectrum based on the corresponding relative wavenumbers ofthe tuning parameters is performed by assigning relative wavenumbers toeach value of the detected gas absorption spectrum, based on theassociated tuning parameter to yield the modified gas absorptionspectrum.
 8. The method of claim 7 wherein the step of comparing themodified gas absorption spectrum with an input gas absorption spectrumto determine corresponding absolute wavenumbers for the tuningparameters includes the steps of: inputting a predetermined gasabsorption spectrum specifying gas absorption as a function of absolutewavenumber; correlating the modified gas absorption spectrum, which is afunction of relative wavenumber, with the input absorption spectrum,which is a function of absolute wavenumber, to determine an offset ofthe relative wavenumbers from the absolute wavenumbers; and adjustingthe wavenumbers associated with each value of the tuning parameters bythe offset value to thereby provide an absolute wavenumber for eachvalue of the tuning parameters such that the absolute transmissionwavelength of the laser is thereby known as a function of the tuningparameters.
 9. A system for calibrating a laser comprising: means forrouting an output beam from the laser through an etalon while tuning thelaser through a range of discrete laser tuning parameters where saidlaser tuned parameters are selected from the group consisting ofcurrent, voltage, and temperature to produce an etalon transmissionspectrum; means for routing an output beam from the laser through a gascell having a gas absorption spectrum of a gas of known light absorbingcharacteristics while tuning the laser through the range of discretelaser tuning parameters to produce a gas absorption spectrum; means fordetecting and storing peaks of the etalon transmission spectrum and thegas absorption spectrum; and means for comparing the stored detectedetalon transmission spectrum with the detected gas absorption spectrumto determine the absolute transmission wavelength of the laser, therebyproviding references for precisely tuning the laser.
 10. The system ofclaim 9 further including: means for selecting a wavelength fortransmission; means for determining particular tuning parameters neededto tune the laser to the selected transmission wavelength based on theabsolute transmission wavelength of the laser; and means for tuning thetransmission laser to the selected wavelength using the particulartuning parameters.
 11. The system of claim 10 wherein the laser is tunedbased on an input voltage and wherein means for determining theparticular tuning parameters needed to tune the laser to the selectedwavelength operates to determine the input voltage.
 12. The system ofclaim 10 wherein the wavelength for transmission is selected from agroup of predetermined International Telecommunication Industry (ITU)fiber optic transmission grid lines.
 13. The system of claim 9 whereinthe means for comparing the detected etalon transmission spectrum withthe detected gas absorption spectrum to determine the absolutetransmission wavelength of the laser as a function of the laser tuningparameters includes: means for comparing the detected etalontransmission spectrum, which is a function of tuning parameters, withrelative wavenumbers of known etalon transmission lines to determinecorresponding relative wavenumbers for the tuning parameters; means forconverting the detected gas absorption spectrum, which is a function oflaser tuning parameters, to a modified gas absorption spectrum, which isa function of relative wavenumber, based on the corresponding relativewavenumbers of the tuning parameters; and means for comparing themodified gas absorption spectrum, which is a function of relativewavenumber, with an input gas absorption spectrum, which is a functionof absolute wavenumber, to determine corresponding absolute wavenumbersfor the tuning parameters such that the absolute transmission wavelengthof the laser as a function of the laser tuning parameters is therebydetermined.
 14. The system of claim 13 wherein the means for comparingthe detected etalon transmission spectrum with relative wavenumbers ofknown etalon transmission lines to determine corresponding relativewavenumbers for the tuning parameters includes: means for identifyingtransmission lines in the detected etalon transmission spectrum; meansfor assigning a relative wavenumber to the tuning parameter valueassociated with each consecutive etalon transmission line; and means forinterpolating between the transmission lines to assign relativewavenumbers to each intermediate value of the tuning parameters suchthat the corresponding relative wavenumbers for the tuning parametersare thereby determined by the relative wavenumbers.
 15. The system ofclaim 14 wherein the means for converting the detected gas absorptionspectrum to the modified gas absorption spectrum based on thecorresponding relative wavenumbers of the tuning parameters operates byassigning relative wavenumbers to each value of the detected gasabsorption spectrum, based on the associated tuning parameter to yieldthe modified gas absorption spectrum.
 16. The system of claim 15 whereinthe means for comparing the modified gas absorption spectrum with aninput gas absorption spectrum to determine corresponding absolutewavenumbers for the tuning parameters includes: means for inputting apredetermined gas absorption spectrum specifying gas absorption as afunction of absolute wavenumber; means for correlating the modified gasabsorption spectrum, which is a function of relative wavenumber, withthe input absorption spectrum, which is a function of absolutewavenumber, to determine an offset of the relative wavenumbers from theabsolute wavenumbers; and means for adjusting the wavenumbers associatedwith each value of the tuning parameters by the offset value to therebyprovide an absolute wavenumber for each value of the tuning parameterssuch that the absolute transmission wavelength of the laser is therebyknown as a function of the tuning parameters.
 17. A system forcalibrating a laser, the system comprising: an etalon; a gas absorptioncell containing a gas of known light absorption characteristics; awaveguide for routing an output beam from the laser through the etalonand through the gas cell; a control unit tuning the laser through arange of discrete laser tuning parameters where said laser tunedparameters are selected from the group consisting of current, voltage,and temperature while the output beam from the laser is routed throughthe etalon and through the gas cell to produce an etalon transmissionspectrum and to produce a gas absorption spectrum; a detector fordetecting and storing peaks of the etalon transmission spectrum and thegas absorption spectrum; and an absolute transmission wavelengthdetermination unit operative to compare the stored detected etalontransmission spectrum with the stored detected gas absorption spectrumto determine the absolute transmission wavelength of the laser as afunction of the laser tuning parameters, thereby providing referencesfor precisely tuning the laser.
 18. The system of claim 17 furtherincluding: a transmission wavelength input selection unit; and whereinthe absolute transmission wavelength determination unit determines theparticular tuning parameters needed to tune the laser to a selectedtransmission wavelength based on the absolute transmission wavelength ofthe laser and wherein the control unit operates to tune the transmissionlaser to the selected wavelength using the particular tuning parameters.19. The system of claim 17 wherein the absolute transmission wavelengthdetermination unit includes: a relative wavenumber determination unitoperative to compare the detected etalon transmission spectrum withrelative wavenumbers of known etalon transmission lines to determinecorresponding relative wavenumbers for the tuning parameters; a gasabsorption spectrum conversion unit operative to convert the detectedgas absorption spectrum to a modified gas absorption spectrum, which isa function of relative wavenumber, based on the corresponding relativewavenumbers of the tuning parameters; an gas reference input unitoperative to input a reference absorption spectrum for the gas as afunction of absolute wavenumber, and a gas reference comparison unitoperative to compare the modified gas absorption spectrum with the inputgas absorption spectrum to determine corresponding absolute wavenumbersfor the tuning parameters.
 20. The system of claim 19 wherein therelative wavenumber determination unit operates to identify transmissionlines in the detected etalon transmission spectrum, assign a relativewavenumber to the tuning parameter value associated with eachconsecutive etalon transmission line, and interpolate between thetransmission lines to assign relative wavenumbers to each intermediatevalue of the tuning parameters to determine corresponding relativewavenumbers for the tuning parameters.
 21. The system of claim 19wherein the gas absorption spectrum conversion unit operates to assignrelative wavenumbers to each value of the detected gas absorptionspectrum based on the associated tuning parameter to yield the modifiedgas absorption spectrum.
 22. The system of claim 21 wherein the gasreference comparison unit operates to correlate the modified gasabsorption spectrum with the reference absorption spectrum to determinean offset of the relative wavenumbers from the absolute wavenumbers andadjust the wavenumbers associated with each value of the tuningparameters by the offset value to thereby provide an absolute wavenumberfor each value of the tuning parameters.
 23. A method for calibrating alaser using an automatically-tunable etalon and a gas cell having aknown gas, said method comprising the steps of: calibrating absolutetransmission wavelengths of the laser using etalon transmission lines ofthe automatically-tunable etalon and gas absorption lines for the knowngas in the gas cell; tuning the laser to align a selected transmissionwavelength of the laser to a known telecommunications transmissionwavelength; tuning the etalon with a microcontroller to align atransmission line of the etalon to the selected transmission wavelength;detecting any drift between the tuned etalon transmission line and theselected transmission wavelength; and re-tuning the laser to compensatefor any drift between the tuned etalon transmission line and theselected transmission wavelength to thereby lock a wavelength of anoutput beam of the laser to the selected transmission wavelength. 24.The method of claim 23 wherein the step of calibrating the absolutetransmission wavelengths of an automatically-tunable etalon using gasabsorption lines for a known gas includes the steps of: routing anoutput beam from the laser through the etalon while tuning the laserthrough a range of tuning parameters to produce an etalon transmissionspectrum as a function of the laser tuning parameters; routing an outputbeam from the laser through a gas cell containing the known gas whiletuning the laser through the range of tuning parameters to produce a gasabsorption spectrum as a function of the laser tuning parameters;detecting the etalon transmission spectrum and the gas absorptionspectrum; and comparing the detected etalon transmission spectrum withthe detected gas absorption spectrum to calibrate the absolutetransmission wavelength of the etalon as a function of the etalon tuningparameters.